What number comes next in a sequence

Question

What number comes next in sequence 3, -6, 12, 4, 20, ?

 

Answer 1

This is a little tricky, but I think it's 13.

 

It appears to be:   3 (-9) -6 (+18) 12 (-8) 4 (+16) 20   for a pattern of -x, then +2x, followed by -(x-1), then +2(x-1), which should be followed by -(x-2), then +2(x-2), where you are starting with x = 9,

so the next number should be  -(9-2), or (-7).

 

To solve these you write the sequence  3    -6    12  4  20    then determine the relationship. (at first this looks like times -2 then times +2 but that falls apart at 4. But by finding the difference between the numbers, the pattern emerges.

 

                                     3        -6         12       4          20        13          27

                                         -9       +18       -8      +16         -7        +14